In a metal oxide, Pb(Zr,Ti)O3 (PZT: lead zirconate-titanate), which has a Perovskite type structure, is a ferrodielectric material and is often used in fields of an actuator, a pressure sensor, etc., using piezoelectric characteristics as a ferrodielectric. Moreover, a PZT thin film, which may be used for various usages such as a non-volatile memory, a piezoelectric device, an optical device, etc., is highly versatile.
As known ferrodielectric materials, there is the above-described PZT, which is a Pb-containing Perovskite type ferrodielectric, and a composite metal oxide such as SrBi2Ta2O9(SBT), etc., which is a Bi-containing layer-structured ferrodielectric. In general, a film made of such a ferrodielectric material is usually formed by MOCVD (metal organic chemical vapor deposition) or sputtering (see Non-patent document 1, for example).
However, a large-sized apparatus including an exhaust system is needed in MOCVD and sputtering. Moreover, when it is sought to form a ferrodielectric film into a desired shape, it is necessary to form the ferrodielectric film, then form a resist pattern on the ferrodielectric film, and then have the resist pattern undergo dry etching by RIE (reactive ion etching), etc. Therefore, there is a tendency for the number of steps to become large and the manufacturing cost to become high.
Therefore, in recent years, a simple and easy low-cost method of manufacturing a metal oxide film using a liquid phase process such as a sol-gel method, from which a decreased manufacturing cost is expected is being investigated. In the sol-gel method, first, an organic metal compound to be a raw material of the metal oxide film is dissolved in a solvent made of an organic material, etc., and a network structure of a metal element and oxygen is formed by hydrolysis and condensation reaction to produce a precursor solution. In the method, the metal oxide film is then formed by applying, etc., a precursor solution in sol state on a substrate by spin coating or dip coating (see Patent document 1, Non-patent document 2).
Now, when the metal oxide film is formed on the substrate by the sol-gel method, in a process such that the precursor solution applied, etc., stiffens and becomes the metal oxide film, a detachment of an organic group by the hydrolysis and the condensation reaction and a shrinkage by volatilization of the solvent occur, leading to a likelihood of cracking, etc., occurring in the metal oxide film. Therefore, in order to form the metal oxide film of a desired film thickness, it is necessary, that, with a thickness of a film made of the precursor solution applied, etc., for each round being set to be no greater than 100 nm, a step of applying, etc., the precursor solution and a step of drying and provisional burning be repeated multiple times, and then a step of non-provisional burning be performed at the end. Moreover, for producing a device, etc, which has the metal oxide film, it is usually necessary to form a metal oxide film of at least 1 μm into a desired shape. However, as the metal oxide film has high dry etching resistance, or in other words, an etching rate of the metal oxide film is relatively slow, time is required when forming the metal oxide film into the desired shape, leading to high cost (see Patent document 1, for example).
Moreover, in order to form the ferrodielectric film made of the metal oxide by the sol-gel method, it is necessary to crystallize the metal oxide film, so that a thermal process at a high temperature is conducted. For example, a thermal process at around 700° C. is conducted for forming a PZT film, while a thermal process at around 800° C. is conducted for forming an SBT film. The thermal process for crystallizing such a metal oxide film is usually conducted by heating the whole substrate with a quartz heating furnace, etc., (see Patent Document 1 and Non-patent document 2, for example).
However, the substrate deforms, etc., at a temperature of 500° C. or above for a glass substrate and at a temperature of 200° C. or above for a plastic substrate, so that it is not preferable to heat the whole substrate in order to crystallize the metal oxide film depending on a material which makes up the substrate. Thus, there is a demand for crystallizing the metal oxide film at a temperature of below 500° C. for the glass substrate and at a temperature of below 200° C. for the plastic substrate.
Therefore, as a method of forming the metal oxide film which is crystallized at a low temperature, there is laser anneal, in which laser is irradiated on the metal oxide film formed so as to crystallize the formed metal oxide film; however, even in this case, cracking, etc., are likely to occur as the metal oxide film shrinks due to shrinkage by the laser light irradiation. Moreover, in order to form the metal oxide film into a desired shape, it is necessary to conduct forming of the resist pattern and etching, etc., by RIE, etc., so that it is not possible to reduce the number of steps.